The present invention pertains to a borehole tool, and more particularly to a device for initiating the operation of such a tool upon its reaching an operating position in the borehole.
When making drill holes in the ground, particularly drill holes which run partly through rock and partly through looser earth layers, it often happens that the dip or inclination of the drill hole to the horizontal plane as well as the direction of the hole will vary quite considerably at different depths of the hole. In many cases, therefore, it is important to determine the existing deviations from a desired inclination and direction.
One such apparatus which has been used for the purposes outlined above is termed a single shot magnetic directional survey instrument. Such an instrument is used to obtain a single record of the inclination and the direction of inclination at various depths in a borehole. The instrument is used for the purpose of orienting a hole in directional drilling operations, and also to chart the course of boreholes from surface to total depth during a normal drilling operation. The instrument includes a magnetic compass and a form of inverted plumbob. These two features are combined into a single compass angle unit which may be available in various ranges of degrees of inclination. A camera unit in the instrument makes a permanent record of the compass angle unit reading at a preset moment in time. Electrical power to the system and to operate lamps in the camera unit may be furnished by batteries or by a conductor cable suspending the instrument from the earth's surface. An electrical circuit for activating the lamps is controlled by a clock which allows a predetermined exposure period of a film disc for recording the inclination and direction of inclination indicated by the compass angle unit. An adjustment feature on the clock typically allows the clock to be set for delayed times ranging from one minute to several hours.
In operating such an apparatus, a desired compass angle unit is selected and assembled, and the batteries and film are loaded with the clock being set for the delayed time needed to reach the survey depth. The instrument is then assembled into a protective casing which can be lowered and positioned in a non-magnetic drill collar at the lower end of the drill stem with the apparatus being suspended on a wire line. Alternatively, the apparatus may be go-deviled into a non-magnetic drill collar and retrieved with an overshot or by pulling the drill string. The instrument may also be lowered into an open hole on a wire line, or by any other convenient method. The compass angle unit is normally comprised of a floating magnetic needle designed so that its directional and angular position may be simultaneously photographed on a disc.
Clock operated survey instruments must be assembled and programmed just prior to the survey since the clock must be set just before the instrument is placed in the well. This is due to the many contingent features that may occur in the drilling of a well and in the operation of equipment for placing the instrument at the surveying location in the wellbore, which might delay the surveying of the well.
In order to set an appropriate delay time in the timing mechanism of the instrument, an estimation must be made as to the time it will take the instrument to be assembled and then either lowered into the wellbore on a wire line, or go-deviled to the bottom. In any event, since the time lapse cannot be predicted with precise certainty, a liberal safety factor must be employed to insure that sufficient time will elapse to permit the instrument to be positioned at the bottom of the borehole or at survey depth. In addition, time must be permitted for the instrument to cease moving or become quiet so that an accurate survey may be performed. Upon lapse of the predetermined time, a lamp circuit is activated which exposes the film to record an image of the compass angle unit.
Since a liberal safety factor must be employed in setting the time for the instrument, a considerable amount of rig time may be used in the running of such a survey. If a directional hole is being drilled for example, and several surveys are run in the course of a day to determine the orientation of the hole, the rig time expended may become very expensive, particularly in situations where rig costs are extremely high such as in offshore operations.
Another approach to the problem of actuating a single shot magnetic directional survey instrument in its proper downhole position is disclosed in U.S. Pat. Nos. 3,638,235, 3,546,478, and 3,588,908. In these patents, the absence of motion of the instrument is sensed to indicate that it is at its proper downhole operating position. One problem encountered with this approach is that occasionally the instrument is loaded and set for use and then inadvertently allowed to lay idle before placement in the borehole for a sufficient amount of time to permit its premature operation. As a result, the film disc which records the inclination is exposed before the instrument is even inserted into the borehole. This results in much wasted time and very high expenses as the operator utilizes all of the time normally required to take a measurement only to find out later, when the film is exposed, that the measurement is erroneous. An indication that the instrument has been operated before insertion into the well is therefore a desirable feature. Additionally, the instruments described in the foregoing patents are all activated by a lack of sensed motion. Under certain circumstances, for example if total absence of motion is not attainable, it may be desirable to operate the instrument solely in response to a lapsed period of time while at other times it may be preferable to operate the instrument upon the absence of motion for a predetermined period of time or a combination of these modes. In any event the option to choose between these modes of operation is a desirable feature in a borehole instrument such as a survey tool.
Another circumstance which can cause wasted time and money is related to the malfunctioning of the motion sensing device and associated electrical circuitry. Should such a malfunction occur and be undetected, an abortive survey is likely. Consequently, it is desirable to provide a means for checking the operability of this function just prior to use of an instrument at the well site.
It is therefore an object of the present invention to provide a new and improved instrument which has selective modes of operation and which eliminates unnecessary delay times presently involved in operating downhole surveying instruments, together with a means of checking functional operating characteristics, resulting in an instrument which is more reliable in operation and more economical to use.